Programmed death ligand 1 (PD-L1) is a 40 kDa type 1 transmembrane protein. PD-L1 (human PD-L1 cDNA is composed of the base sequence shown by EMBL/GenBank Acc. No. NM_001267706 and mouse PD-L1 cDNA is composed of the base sequence shown by NM_021893) that is a ligand of PD-1 is expressed in so-called antigen-presenting cells such as activated monocytes and dendritic cells. These cells present interaction molecules that induce a variety of immuno-inductive signals to T lymphocytes, and PD-L1 is one of these molecules that induce the inhibitory signal by ligating PD-1. It has been revealed that PD-L1 ligation suppressed the activation (cellular proliferation and induction of various cytokine productions) of PD-1 expressing T lymphocytes. PD-L1 expression has been confirmed in not only immunocompetent cells but also a certain kind of tumor cell lines (cell lines derived from monocytic leukemia, cell lines derived from mast cells, cell lines derived from hepatic carcinomas, cell lines derived from neuroblasts, and cell lines derived from breast carcinomas) (Nature Immunology (2001), vol. 2, issue 3, p. 261-267.).
Programmed death 1 (PD-1) is a member of the CD28 family of receptors, which includes CD28, CTLA-4, ICOS, PD-L1, and BTLA. The initial member of the family, CD28, was discovered by functional effect on augmenting T cell proliferation following the addition of monoclonal antibodies (Hutloff et al. (1999) Nature 397:263-266; Hansen et al. (1980) Immunogenics 10:247-260). Two cell surface glycoprotein ligands for PD-1 have been identified, PD-L1 and PDL-2, and have been shown to down-regulate T cell activation and cytokine secretion occur upon binding to PD-1 (Freeman et al. (2000) J. Exp. Med. 192:1027-34; Latchman et al. (2001) Nat. Immunol. 2:261-8; Carter et al. (2002) Eur. J. Immunol. 32:634-43; Ohigashi et al. (2005) Clin. Cancer Res. 11:2947-53). Both PD-L1 (B7-H1) and PD-L2 (B7-DC) are B7 homologs that bind to PD-1. Expression of PD-L1 on the cell surface has also been shown to be upregulated through IFN-γ stimulation.
PD-L1 expression has been found in several murine and human cancers, including human lung, ovarian and colon carcinoma and various myelomas (Iwai et al. (2002) Proc. Natl. Acad. Sci. USA 99:12293-7; Ohigashi et al. (2005) Clin. Cancer Res. 11:2947-53). PD-L1 has been suggested to play a role in tumor immunity by increasing apoptosis of antigen-specific T-cell clones (Dong et al. (2002) Nat. Med. 8:793-800). It has also been suggested that PD-L1 might be involved in intestinal mucosal inflammation and inhibition of PD-L1 suppresses wasting disease associated with colitis (Kanai et al. (2003) J. Immunol. 171:4156-63).